Cranial nerves (Types, Origin, Distribution and Function)

• The nerves that arise from the brain and supply to the head, neck and face are called cranial nerves.
• There are 12 pairs of cranial nerves which are the parts of peripheral nervous system.
• Their names are an indication of some of their anatomical or functional features, and their numbers (Roman numerals) indicate the sequential order in which they emerge from the brain.
• Cranial nerves I and II are nerves of the cerebrum, nerves III to XII are nerves of the brainstem (XI partly emerging from spinal cord).
• Of the 10 brainstem nerves, 1 (VIII) is a purely sensory nerve, 5 (III,IV,VI,XI and XII) are primarily motor nerves and 4 (V,VII,IX and X) are mixed nerves, i.e. containing both sensory and motor fibers.
• The cranial nerves are generally concerned with the specialized (special) senses of smell, taste, vision, hearing and balance, and with the general senses.
• They are also involved with the specialized motor activities of eye movement, chewing and swallowing, breathing, speaking and facial expression.
• The 12 pairs of cranial nerves are as follows:

Cranial nerve I (Olfactory nerve):

1. Type: sensory nerve
2. Origin: nasal mucous membrane high in the nasal cavities.
3. Distribution: terminates in the olfactory bulb of cerebrum
4. Function: perception of smell or olfaction

Cranial nerve II (Optic nerve):

1. Type: sensory nerve
2. Origin: retina of the eye
3. Distribution: terminates in lateral geniculate body of thalamus and superior colliculus of midbrain
4. Function:
   • perception of vision
   • adjusts lens and constricts pupil helping in the reflex of focusing

Cranial nerve III (Oculomotor nerve):

1. Type: motor nerve
2. Origin: midbrain
3. Distribution:
   • to all extrinsic muscles of eyeball except superior oblique and lateral rectus
   • in the autonomic fibers to ciliary muscles of lens and constrictor muscle of iris
4. Function:
   • movements of eyeball, elevation of upper eyelid
   • constriction of pupil
   • accommodation of the lens (focusing by the lens)

Cranial nerve IV (Trochlear nerve):

1. Type: motor nerve
2. Origin: caudal midbrain
3. Distribution: innervates special oblique muscles of eye
4. Function: eye movements (down and out)

Cranial nerve V ( Trigeminal nerve):

• This nerve consists of three major branches and is the largest cranial nerve.
• It is a mixed nerve, sensory nerve of the face, oral and nasal regions, and motor nerve of the chewing muscles.

1. The ophthalmic nerve (V¹):
   • Sensory nerve
   • Originates and supplies to the general area of forehead and eyes
   • It conveys general senses from cornea of eyeball, upper nasal cavity, front of scalp, forehead, upper eyelid, conjunctiva and lacrimal (tear) glands.
2. The maxillary nerve (V²):
   • Sensory nerve
   • Originates and supplies to the general area of maxillary region
   • Conveys general senses from cheek, upper lip, upper teeth, mucosa of nasal cavity, palate and parts of pharynx
3. The mandibular nerve (V³):
   • Mixed nerve (both sensory and motor)
   • Sensory branch originates and supplies to the general area of mandibular region
   • It conveys general senses from tongue (not taste), lower teeth and skin of lower jaw
   • Motor branch originates in the pons and supplies to the muscles of mastication
   • It helps in chewing

Cranial nerve VI (Abducens nerve):

1. Type: motor nerve
2. Origin: caudal pons
3. Distribution: innervates the lateral rectus muscle of eye
4. Function: helps in the abduction of the eye, i.e. lateral movements of the eye

Cranial nerve VII (Facial nerve):

1. Type: mixed nerve
2. Origin: pons
3. Distribution and function:
   • sensory branch innervates the taste buds of tongue and helps in the perception of taste of food
   • motor branch innervates the muscles of facial expression, autonomic fibers to salivary glands, and lacrimal glands
   • it helps in salivation, lacrimation (tear production), movement of muscles of facial expression

Cranial nerve VIII (Vestibulocochlear nerve):

• It is a sensory nerve composed of two nerves; the cochlear nerve and the vestibular nerve

1. The cochlear nerve:

• Sensory nerve
• Originates and supplies to the cochlea of inner ear
• It helps in the perception of hearing

2.The vestibular nerve:

• Sensory nerve
• Originates and supplies to the semicircular ducts, utricle and saccule of inner ear
• It helps in maintaining the body balance or equilibrium.

Cranial nerve IX (Glossopharyngeal nerve):

1. Type: mixed nerve
2. Origin: medulla oblongata
3. Distribution and function:
   • Sensory branch originates and supplies to the posterior third of tongue and upper pharynx
   • It helps in the perception of taste and other sensations of the tongue along with general senses from upper pharynx
   • Motor branch originates and supplies to the stylopharyngeus muscle and autonomic fibers of parotid gland.
   • It helps in the secretion of saliva and swallowing of food.

Cranial nerve X (Vagus nerve):

1. Type: mixed nerve
2. Origin: medulla oblongata
3. Distribution: innervates the voluntary muscles of soft palate, cardiac muscle, smooth muscle in respiratory, cardiovascular and digestive system
4. Function:
   • helps in swallowing
   • monitors the oxygen and carbon dioxide concentrations in blood
   • Senses blood pressure and other visceral activities of affected systems.

Cranial nerve XI (Accessory nerve):

1. Type: it is a spinal accessory nerve and is a motor nerve
2. Origin: medulla oblongata and cervical spinal cord
3. Distribution: muscles of larynx, strenocleidomastoid and trapezius muscles
4. Function: voice production from larynx, muscle sense, and movement of the head and shoulders

Cranial nerve XII (Hypoglossal nerve):

1. Type: motor nerve
2. Origin: medulla oblongata
3. Distribution: innervates the tongue muscles
4. Function: movements of tongue during speech, helps in swallowing and muscle sense

Cranial nerves (Types, Origin, Distribution and Function)